Optical waveguiding structures incorporating embedded hollow microchannels and microchannel arrays are used in a variety of applications, ranging from waveguides in three-dimensional (3D) and two-dimensional (2D) photonic bandgap materials, or photonic crystals, to microfluidic systems for optical detecting and testing microscopic quantities of organic and non-organic molecules in liquids, for monitoring biochemical reactions, for use in lab-on-a-chip devices and chemical and environmental sensors. Integrated systems including microfluidic channels and optical waveguides can also be of interest for optical devices for controlling and generating light, such as lasers, optical modulators, switches etc.
Photonic crystals are artificially structured materials consisting of 3D or 2D periodic structures with typical periods ranging from about a micron down to hundreds of nanometers, which give rise to photonic band gaps affecting propagation of light similarly to the way periodic potentials in semiconductor or dielectric crystals affect the motion of electrons by defining allowed and forbidden electronic energy bands. Photonic bandgap materials can enable a range of novel optical devices and functions such as low-loss waveguiding in sharp waveguide bends, high-reflectivity omnidirectional mirrors and inhibition of spontaneous emission.
Fabrication of photonic band gap structures in silicon-compatible material systems is of especial interest for many applications because it enables an easy integration of such structures with conventional, such a silica-on-silicon planar waveguides. Such fabrication typically requires a tightly controlled 2D and 3D patterning processes for creating 2D or 3D arrays of voids in the material. While for obtaining a true photonic crystal, wherein light propagation is prohibited in all three dimensions for certain wavelengths, fabrication of true 3D arrays of micro-voids is generally required, such fabrication can be prohibitively expensive. Fortunately, for many applications having 2D or even 1D arrays of microchannels embedded in an optical material, or even individual embedded microchannels coupled to optical waveguiding structures, can be sufficient.
In planar silicon-based materials, 2D photonic crystals have typically been fabricated by forming microchannel arrays wherein the microchannels are oriented in vertical direction relative to the structure, i.e. normally to its main planar surface. Planar waveguides can be formed in such structures by forming an optical path in the structure wherein the microchannels are absent, so to guide light in a direction normal to the microchannels surrounding the waveguide. On the other hand, photonic band gap optical fibers, also known as “holey” optical fibers, have hollow micro-channels which are oriented along the waveguiding direction of the finer, generally parallel to the fiber's core. In holey fibers, such orientation of the voids enables a range of useful optical functions, including single mode operation over a wide wavelength range, polarization control, dispersion compensation, transmission of high optical power etc. However, to the best of the inventor's knowledge, no planar waveguiding structures with photonic band gaps have been disclosed wherein the microchannels are oriented in-plane with the substrate, along the waveguiding direction.
Planar structures integrating optical waveguides and enclosed microchannels oriented in-plane with a substrate are known in microfluidic applications, e.g. for optical probing of microscopic amounts of fluids delivered into the microchannels. Such integrated waveguide-microchannel structures can enable also incorporation of active materials into silicon or silica-based materials for fabrication of active photonic devices such as optical modulators, switches, lasers and amplifiers. Various method for fabricating such microchannels for microfluidic devices have been disclosed in the art, including methods that enable their incorporation in planar silicon structures with optical waveguides. These prior art methods typically include formation of open microchannels in a surface layer of a silicon-based planar structure, and a step of bonding a cover plate onto said surface to form one or more enclosed microchannels from the open microchannels. The enclosed microchannels formed this way are typically oriented to cross a waveguide, so to create one or more intersection points wherein a waveguiding mode interacts with a fluid within the enclosed microchannel. U.S. Pat. No. 6,438,279, issued to Craighead, et al., discloses such fabrication techniques for forming microcapiliary and waveguide structures.
However, these prior-art method for formation of enclosed microchannels crossing waveguides have several drawbacks. The additional step of bonding or gluing a top cover plate technique is outside of common commercial techniques of silicon processing, and complicates the fabrication process. Microchannel structures fabricated using this process are limited to a single layer. The microchannels are typically located at the surface, are difficult to seal and fragile. The use of single-point liquid-optical field interaction, wherein the fluid interacts with an optical mode only in points of microchannel-waveguide interactions, greatly limits the interaction length and thus reduces beneficial effects of such interactions. Also, the cover-plate techniques typically provide microchannels which have substantially trapezoidal or rectangular cross-sections with shapes which are difficult to control.
It would therefore be advantageous to have a method of fabricating integrated microchannel-waveguide structures using only conventional techniques of silicon processing, such as photolithographic patterning, etching and silica deposition, which does not use the cover-plate bonding step, enables tightly controlled co-fabrication of embedded microchannels and waveguides in co-linear orientation for increasing the useful fluid-optical mode interaction length, wherein the microchannels and waveguides are co-fabricated controllably close to each other to enable their optical coupling through evanescent field interaction. It would also be advantageous if the same method would enable fabrication of 1D and 2D arrays of microchannels having substantially circular or elliptical cross-sections with tightly controlled parameters, e.g. for applications wherein optical waveguides are integrated with photonic band-gap structures.
An object of the present invention is to provide a method for controlled fabrication of planar waveguiding structures with embedded microchannels having substantially circular or elliptical cross-sections in silicon-based glass materials.
Another object of the present invention is to provide a method for fabricating waveguiding structures comprising uniform 1D and 2D arrays of embedded microchannels with controlled microchannel parameters.
Another object of the present invention is to provide a method for fabricating monolithic waveguiding structures comprising embedded microchannels in close proximity to ridge waveguides, wherein the ridge waveguides and the embedded microchannels are fabricated in parallel using a same technological process.
Another object of the present invention is to provide a simplified method of fabrication of coupled waveguide-microchannel structures for microfluidic applications without using the cover plate bonding or gluing step for enclosing the microchannels.